1. Field of the Invention
The present invention relates to a pharmaceutical composition for treating liver diseases, comprising a miRNA mimic containing a single strand RNA molecule of hsa-miR-21-3p (SEQ ID No: 35). In addition, the present invention also relates to a method for treating liver diseases and reducing the expression of specific enzymes.
2. Description of the Related Art
Nonalchoholic fatty liver disease (NAFLD) is rapidly becoming one of the most common liver disease because of growing prevalence of overweight and obesity. Generally, NAFLD is defined by fat accumulation, mainly triglycerides, in hepatocytes exceeding 5% of its weight. In the progress of NAFLD, intrahepatic lipid accumulation and growth of lipid droplets result in different degrees of inflammation, thereby resulting in liver fibrosis. As the clinical pathologic spectrum, liver fibrosis may progress advanced cirrhosis, hepatocellular carcinoma, hepatic decompensation, and have increased all-cause mortality. However, there is no standard drug treatment or specific therapy to reverse fatty liver disease. Nowadays, researchers are going to uncover what processes may trigger fat build-up in the liver and how to prevent and treat the fatty liver disease.
Methionine adenosyltransferase (MAT) is the cellular enzyme that catalyzes the synthesis of S-adenosyl methionine (SAM), the principal biological methyl donor and a key regulator of hepatocyte proliferation, death and differentiation[1,2]. Two genes, MAT1A and MAT2A, encode 2 distinct catalytic MAT isoforms. A third gene, MAT2B, encodes a MAT2A regulatory subunit. MAT1A is specifically expressed in the adult liver, whereas MAT2A is widely distributed[3-5]. Because MAT isoforms differ in catalytic kinetics and regulatory properties, MAT1A-expressing cells have considerably higher SAM levels than do MAT2A-expressing cells[6,7]. In hepatocellular carcinoma (HCC), the down-regulation of MAT1A and the up-regulation of MAT2A occur, which is known as the MAT1A:MAT2A switch[8-11]. The switch accompanied with up-regulation of MAT2B results in lower SAM contents, which provide a growth advantage to hepatoma cells[2,4,6,12,13]. SAM can selectively induce pro-apoptotic Bcl-Xs in hepatoma cells, but not in normal hepatocytes, through alternative splicing[14]. In addition, increased MAT2B expression in HCC also results in decreased SAM levels and facilitates cancer cell growth[15]. Because MAT2A and MAT2B play crucial role in facilitating the growth of hepatoma cells, they are valid targets for antineoplastic therapy. Recent studies have shown that silencing MAT2A and MAT2B by using small interfering RNA substantially suppress growth and induce apoptosis in hepatoma cells[16-19].
Acetyl-CoA-carboxylase, which catalyses the carboxylation of acetyl-CoA to form malonyl-CoA, exists in 2 isoforms (alpha and beta) that are separately encoded by ACACA and ACACB in mammals. ACACA, a cytosolic enzyme, is the first committed step of fatty acid synthesis in lipogenic tissue[19]. Carnitine-palmitoyl-CoA transferase I (CPT1), a rate-limiting enzyme that shuttles long-chain fatty acyl-CoAs into the mitochondria for oxidation, is rapidly inhibited by the ACACB-produced malonyl-CoA[20, 21]. Diglyceride acyltransferase (DGAT), the terminal and the only committed enzyme in the biosynthesis of triacylglycerol, plays a key role in hepatic lipid droplet accumulation[22, 23]. There are 2 forms of diglyceride acyltransferase which are separately encoded by DGAT1 and DGAT2. Recent studies have shown that fatty liver disease can be ameliorated or reversed by reducing the expression of ACACA, ACACB, or DGAT2, indicating that pharmacologically inhibiting these genes could be a suitable approach to treating of NAFLD[24, 25].
Berberine is an isoquinoline alkaloid isolated from various medicinal herbs such as Coptis chinensis, and it has a wide range of pharmacological effects including anti-cancer, anti-microbial, anti-inflammatory, and anti-diabetic effects[28-29]. Recent studies have focused on its anti-tumor effects, including anti-proliferation, anti-invasion and apoptosis induction in broad tumor cell types[29-38]. In HCC, berberine has been reported to inhibit cell growth and survival through cell cycle arrest and the activation of autophagic and mitochondrial apoptotic cell death[39-41]. In addition, some reports have shown that berberine has hypoglycemic, hypolipidemic and LDL-lowing effects, and animal studies have proved that berberine reduces the liver fat content in vivo.
MicroRNAs (miRNAs) are small non-coding RNA molecules composed of 21-23 nucleotides that play a critical role in a wide variety of biological processes, including development, proliferation, and death[42, 43]. The deregulated expression of miRNAs is observed in numerous human cancer types, and they can act as tumor suppressors or oncogenes in the tumorigenic process[44, 45]. Mature miRNAs typically direct their posttranscriptional repression by pairing the seed region of the miRNA to 3′ UTRs, the non-coding sequence at the 3′ end of target genes, leading to mRNA destabilization and translational silencing[46,47]. The seed region of miRNAs locates at the 5′ end, from the second to eighth nucleotide. When the seed region pairs with the 3′ UTR of the target gene, it silences the gene. It is not necessary for the miRNA being completely complementary to the 3′ UTR of the target gene. The processing of the precursor miRNA (pre-miRNAs) hairpin generates an miRNA duplex consisting of a guide strand and a passenger strand (also termed as miRNA and miRNA*). By convention, a guide strand is selectively loaded onto an Argonaute (AGO) protein to form an miRNA-induced silencing complex (miRISC), and the passenger strand is believed to be preferentially degraded because of its lower steady-state level[48]. However, current research shows that numerous miRNA* species accumulate to substantial levels, and endogenous miRNA genes do not universally exclude miRNA* species from functional miRISC complexes, which suggests that miRNA* species should be considered[49-54].